x-pack/filebeat/input/cel: update mito version to v1.6.0

[efd6]

Proposed commit message

See title.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• This picks up a bugfix in JSON encoding. lib: fix json_encode for null values mito#44

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Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

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• Start Time: 2023-09-22T04:58:25.613+0000

• Duration: 139 min 4 sec

Test stats 🧪

Test	Results
Failed	0
Passed	28277
Skipped	2013
Total	30290

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